Within the semiconductor industry, both package level and device level hermetic sealing is employed to create, enable, and ensure a necessary and stable operating environment. For instance, many Micro Electrical Mechanical Systems (MEMS) employ multiple device wafers joined together into a functional stack that require a hermetically sealed environment. In particular, micro-sensor devices that are formed in silicon device wafers are often protected from the atmosphere by bonding a cap wafer to the top surface of the device wafer. When the bond is not complete the seal is compromised and there is a leak path into the interior of the device allowing air, moisture and other contaminants to enter through the leak path. If the seal is compromised, the device may not function optimally and even may be rendered completely inoperable. For devices requiring a vacuum or sub-atmospheric pressure, loss of seal integrity allows entry of air causing inaccurate operation or loss of functionality of the device. Moreover, moisture in the incoming air can cause moving parts to stick together permanently and may freeze at low temperatures impeding movement of mechanical portions of the device preventing necessary response to mechanical or electrical stimuli. Particulates and packaging material in the incoming air, such as protective, stress-relieving, or dielectric gels can also interfere with the device operation.
Inevitably, some seals are compromised or malformed during the manufacturing process. It is most economical to test hermeticity as soon as possible after the seal is made. It follows then that for device level sealing a device level test is desirable and that for package level sealing a package level test prior to integration onto a printed circuit board (PCB) or other device wafer is desirable. Such tests ensure that defective devices are not shipped and may avoid further costly processing of defective devices.
Processes for seal leak detection are already known. U.S. Pat. No. 6,074,891 granted to Steven Edward Staller Jun. 13, 2000, discloses a process and semiconductor device for verifying a hermetic seal that uses an unpassivated junction diode for leak detection. The reverse diode characteristics of a junction diode of proper size become unstable when moisture or liquid water is present. Bonding of a cap wafer to a device wafer often takes place in a controlled atmosphere and/or in a vacuum. Theoretically any device comprising a device wafer and a cap wafer that is not hermetically sealed may be detected electronically at wafer test when there is water vapor present in the air. According to the Staller '891 patent, water vapor in air of greater than about 40% relative humidity causes measurable instability in the junction diode.
In high volume testing conditions, sealing a cap wafer to a device wafer in air of variable humidity occasionally allows a leaky device to escape detection. Thus in a modified process, liquid water is forced into any improperly sealed device to assure an unstable unpassivated junction diode. Furthermore, when the leak path is large, the liquid water that was forced into the leaky device may drain out allowing the device cavity to dry out, so that the unpassivated junction diode may not be measurably instable. Thus, an unsealed device may not be identified at wafer test. Consequently, there is a time constraint between the water soak and the wafer test and an additional inspection of the cap wafers is often required to help contain devices with large holes that may not be detected at wafer test.
Further complicating leak detection, sensor device sizes are shrinking because economies of scale allow for similar functionality on smaller and smaller devices. It follows, then, that the volume within the cavity of a sensor device are getting smaller as well. A smaller volume holds less water on and reduces the amount of time the bare or unpassivated junction diode is able to detect a leaky device. A reduced device size does not shorten the necessary electrical functional test or the time required to tests each device. Thus a wafer with smaller devices will have a proportionally longer residence time on the tester for the same set of functional tests. This is not insignificant. For instance, a wafer with 50% more devices on the slice takes approximately 50% more time to test, which increases the time that water may be draining out of leaky devices and evaporating from the unpassivated junction diode. This increases the likelihood of a test escape, and significantly tightens the time constraints on the test floor between the water soak and wafer test.
The act of applying a physical stimulus, such as moisture introduction, has several inherent problems in addition to those discussed above. First, the simple task of introducing moisture requires several post process operations atypical of common semiconductor manufacturing flows. This inherently adds cost and complexity to the fabrication process. Secondly, if the stimulation is either not applied or inappropriately applied the junction diode may not be measurably unstable. Again resulting in the risk that a poorly sealed device may not be identified.
A further drawback of the leak detection method disclosed by the Staller '891 patent is that the method requires the reliable formation of an unpassivated junction diode. A manufacturing defect that fails to unpassivate the diode structure is not sensitive to moisture. This in turn causes the structure to respond as if the device is properly sealed. Furthermore, many MEMS structures of interest, capacitive, thermal, etc. simply do not require the use of multiple P or N-type dopant diffusions necessitated by a junction diode. This makes the formation of a junction diode either costly or in some cases virtually impossible limiting the applicability of the Staller leak detection method to a narrow range of sealed devices.
Lastly, the silicon or package area required to support the junction diode detection structure known in the prior art is extensive. In recent years device area for commonly manufactured MEMS has reduced significantly whereas, leak detection structures have not. This makes incorporation of leak detection structure, such as an unpassivated junction diode progressively more expensive.